This invention relates generally to nanoscale fibers, and more particularly to methods for aligning carbon nanotubes or other nanoscale fibers to a high degree of alignment in the production of buckypaper composite materials.
Carbon nanotubes and nanofibers have both rigidity and strength properties, such as high elasticity, large elastic strains, and fracture strain sustaining capabilities. Such a combination of properties is generally not present in conventional materials. In addition, carbon nanotubes and nanofibers are some of the strongest fibers currently known. For example, the Young's Modulus of single-walled carbon nanotubes can be about 1 TPa, which is about five times greater than that for steel (about 200 GPa), yet the density of the carbon nanotubes is about 1.2 g/cm3 to about 1.4 g/cm3. The tensile strength of single-walled carbon nanotubes is generally in the range of about 50 GPa to about 200 GPa. This tensile strength indicates that composite materials made of carbon nanotubes and/or nanofibers could likely be lighter and stronger as compared to current high-performance carbon fiber-based composites.
In addition to their exceptional mechanical properties, carbon nanotubes and nanofibers may provide either metallic or semiconductor characteristics based on the chiral structure of fullerene. Some carbon nanotubes and nanofibers also possess superior thermal and electrical properties such as thermal stability up to about 2800° C. in a vacuum and about 750° C. in air, thermal conductivity about twice as much as that of diamond, and an electric current carrying capacity about 1000 times greater than that of copper wire. Therefore, carbon nanotubes and nanofibers are regarded as one of the most promising reinforcement materials for the next generation of high-performance structural and multifunctional composites.
Thin films or sheets of nanoscale fiber networks, or buckypapers (BP), offer a promising platform to fabricate high-performance nanoscale fiber composites because BPs are easy to handle during fabrication of the composite, and thus, may be incorporated into conventional composites processing to fabricate nanocomposites.
Nanoscale fibers have both exceptional mechanical and functional properties, which conventional macroscopic carbon fibers do not offer. However, four main factors tend to affect the performance of nanocomposites: 1) nanoscale fiber dispersion, 2) nanoscale fiber alignment, 3) interface bonding between the nanoscale fibers and the composite matrix, and 4) aspect ratio of the nanoscale fibers. For instance, the composite nanoscale fiber loading may be too low (less than 20 wt %), there may be a lack of adequate nanoscale fiber alignment, or the smaller aspect ratios of nanoscale fibers such as CNTs (less than 10,000) may result in poor load transfer between the matrix and CNTs when the composites are under loads.
Methods for aligning nanoscale fibers such as carbon nanotubes include magnetic field-induced alignment, mechanical stretching of synthesized nanotube forests, shear force-induced alignment, AC electric field alignment, electrospinning, and electrophoretic alignments during nanotube composite fabrication. However, the loose and weakly bonded structures of nanotube networks make it difficult to uniformly transfer force throughout nanotube networks, thus hindering the development of practical methods to further improve nanoscale fiber alignment in BP through mechanical stretching to achieve a high degree of alignment (e.g., greater than 20%).
It would therefore be desirable to provide improved nanotube alignment techniques for alignment of nanoscale fibers in BP.